Examples of known cutting inserts used for machining a work material such as a metal include throw-away tips described in Japanese Unexamined Patent Application Publication No. 2010-042462A (Patent Document 1) and Japanese Unexamined Patent Application Publication No. 2007-007736A (Patent Document 2). The tip recited in Patent Document 2 has a rhombic shaped top surface and bottom surface, of which one constitutes a rake face and the other constitutes a seating face. A chip breaker is formed on each of the top surface and the bottom surface, toward an acute-angled corner portion and an obtuse-angled corner portion. Here, the chip breaker on the side of the top surface and the bottom surface, serving as the seating face, is a surface that contacts a holder.
When attaching the cutting insert to the holder, the closer the surface of the top surface and the bottom surface that contacts the holder is located to the corner portions of the top surface and the bottom surface, the more stably the cutting insert can be secured to the holder. Particularly, in cases where performing machining using the obtuse-angled corner portion, the behavior of the chips easily becomes unstable due to the thickness of the chips becoming thinner. However, the chips can be processed well by positioning the surface that contacts the holder near the corner portions of the top surface and the bottom surface. Additionally, in cases where performing machining using the acute-angled corner portion, the surface that contacts the holder may come excessively close to the acute-angled corner portion due to the thickness of the chips becoming thicker. In this case, space for curling the chips becomes smaller and, consequently, the processing of the chips may become unstable.
In light of the problems described above, an object of the present invention is to provide a cutting insert that can be stably secured in a holder and can process chips well.